ACTAR Crash Dynamics 2 — Questions and Answers
Question 1: During a head-on collision between two vehicles of equal mass and equal speed, what happens to their combined momentum?
- It doubles
- It remains constant
- It equals zero (Correct answer)
- It is transferred entirely to the heavier vehicle
Correct answer: It equals zero
When two equal-mass vehicles traveling at equal and opposite speeds collide head-on, their momenta cancel, resulting in a net combined momentum of zero.
Question 2: Which term describes the ratio of the relative separation speed to the relative approach speed of two colliding objects?
- Coefficient of friction
- Coefficient of restitution (Correct answer)
- Impact angle ratio
- Velocity overlap factor
Correct answer: Coefficient of restitution
The coefficient of restitution (e) quantifies the elasticity of a collision by comparing post-impact separation speed to pre-impact approach speed.
Question 3: In crash reconstruction, the 'crush stiffness coefficient' (A and B) is used primarily to determine what?
- Skid mark length
- Delta-V from crush depth measurements (Correct answer)
- Tire-road friction
- Airbag deployment threshold
Correct answer: Delta-V from crush depth measurements
The Campbell crush model uses empirically derived stiffness coefficients A and B to relate residual crush depth to the energy absorbed and estimate Delta-V.
Question 4: A vehicle strikes a fixed rigid barrier and comes to a complete stop. If the vehicle's mass is 1,500 kg and its pre-impact speed is 20 m/s, what is its initial kinetic energy?
- 15,000 J
- 150,000 J
- 300,000 J (Correct answer)
- 30,000 J
Correct answer: 300,000 J
Kinetic energy = ½mv² = 0.5 × 1,500 × (20)² = 0.5 × 1,500 × 400 = 300,000 J.
Question 5: What is the primary reason vehicle roof crush is analyzed separately from frontal/side crush in ACTAR reconstructions?
- Roof crush involves different energy absorption mechanics and occupant space intrusion concerns (Correct answer)
- Roof crush only occurs in rollovers and has no momentum component
- Roof materials are always stiffer than side panels
- Roof crush always indicates a secondary collision
Correct answer: Roof crush involves different energy absorption mechanics and occupant space intrusion concerns
Roof crush analysis focuses on structural intrusion affecting occupant survival space, governed by different stiffness characteristics than frontal or side impacts.
Question 6: During an oblique impact, the 'principal direction of force' (PDOF) is best described as:
- The direction of travel of the struck vehicle before impact
- The direction in which the greatest crush force is applied to the vehicle (Correct answer)
- The angle between the two vehicles' headings
- The resultant velocity vector after impact
Correct answer: The direction in which the greatest crush force is applied to the vehicle
PDOF is the direction along which the primary crush force acts on the vehicle during impact, which may differ from the vehicle's pre-impact heading.
Question 7: In a two-vehicle collision reconstruction, if Vehicle A's post-impact trajectory is known and Vehicle B's is known, conservation of momentum allows the reconstructionist to:
- Determine only the direction, not the magnitude, of pre-impact speeds
- Calculate pre-impact speed vectors for both vehicles (Correct answer)
- Confirm airbag deployment times
- Determine the coefficient of restitution only
Correct answer: Calculate pre-impact speed vectors for both vehicles
By applying conservation of momentum with known post-impact velocities and directions, the reconstructionist can solve for the pre-impact speed vectors of both vehicles.
ACTAR Crash Dynamics 2
During a head-on collision between two vehicles of equal mass and equal speed, what happens to their combined momentum?